(CNN) --Amid a storm of controversy over his racially charged comments on a weekend TV show, conservative commentator and radio talk show host Rush Limbaugh announced on Thursday that he had resigned from his post at ESPN.

Limbaugh said on Thursday he stepped down late Wednesday from the show "Sunday NFL Countdown" to protect the network from the uproar caused by his statement that Philadelphia Eagles quarterback Donovan McNabb was overrated because the media wanted to see a black quarterback succeed.

McNabb said Wednesday that he didn't mind criticism of his performance, but was upset that Limbaugh made his race an issue and said it was too late for an apology.

"It's somewhat shocking to hear that on national TV from him," McNabb said. "It's not something that I can sit here and say won't bother me."

On Thursday, Limbaugh told the National Association of Broadcasters at a convention in Philadelphia, "The great people at ESPN did not want to deal with this kind of reaction."

At the convention Limbaugh did not directly address questions about his name being linked by law enforcement sources to an investigation into a black market drug ring in Palm Beach County, Florida.

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The sources said Thursday that authorities are looking into the illegal sales of prescription drugs OxyContin and hydrocodone.

Law enforcement sources said Limbaugh, who has a home in Palm Beach, is not the focus of the investigation. His name came up as a possible buyer, the sources said, during a two-year probe of a suspected multimillion-dollar black market operation.

A spokesman for the state attorney's office in Palm Beach said his office could "not confirm or deny" that Limbaugh's name has come up as part of the investigation.

Limbaugh said in a statement released Thursday: "I am unaware of any investigation by any authorities involving me. No governmental representative has contacted me directly or indirectly. If my assistance is required in the future, I will, of course, cooperate fully."

The statement was issued by Premiere Radio Networks, which syndicates the "Rush Limbaugh Show."